Change in taxonomies in v1.8?

I recently updated a site from 1.4 to 1.8 and have seen a huge performance drop off (3 sec load => 15 sec load) on one of my pages.

I'm wondering if something changed in how taxonomies are handled between the two versions. My page consists of a taxonomy of
categories that has a content picker field for featured
products. Each product is a custom taxonomy term that has a taxonomy field showing which
category it belongs to.

Watching sql profiler it appears that when I load a page displaying the categories they then load the products (as expected), which then loads the category and every product under that category. This amounts to many thousands of rows and is presumably my slowdown.

So, did something change in how taxonomy fields choose to load child objects?

Could you share some traces with us? Are you sure this is coming from the default module and not some custom code?
Would you have a repro with a default setup where you create a custom hierarchy and would let us pinpoint the code path ?

Ran mini-profiler and came up with a confusing (partial) culprit, the logon action takes 4.7 seconds. Nothing is particularly fast, but that surprised me. Not sure why it's firing at all on this page since the user is already logged on.

As to the db rows I mentioned earlier, I was mistaken. A normal-ish number is being requested, the many thousands was from a prior test where there appeared to be a circular reference when two taxonomies each refer to the other.